The present invention relates to a catalytic converter for an automotive exhaust system comprising a metallic casing with a catalyst support (monolith) securely mounted within the casing by a resilient, flexible ceramic fiber containing mounting mat. The mounting mat may be comprised of ceramic fiber alone or preferably is comprised of a composite of ceramic fiber in combination with an intumescent sheet material.
Catalytic converters are universally employed for oxidation of carbon monoxide and hydrocarbons and reduction of the oxides of nitrogen in automobile exhaust gases in order to control atmospheric pollution. Due to the relatively high temperatures encountered in these catalytic processes, ceramics have been the natural choice for catalyst supports. Particularly useful supports are provided by ceramic honeycomb structures as described, for example, in U.S. Pat. Re 27,747.
More recently, catalytic converters utilizing metallic catalyst supports (metallic monoliths) have also been used for this purpose. (See, for example, UK Pat. No. 1,452,982, U.S. Pat. No. 4,381,590 and SAE paper 850131.) The metallic monoliths have better thermal shock resistance and offer lower back pressure due to reduced wall thickness of the monolith forming the gas flow channels.
The metallic monoliths are normally welded or brazed directly onto the outer metallic casing of the catalytic converter which becomes very hot because the heat of the exhaust gas is readily conducted by the metallic monolith to the casing. The high casing temperature can result in undesirable heating of surrounding areas, such as the floorboard and passenger compartment, as well as creating a risk of grass fires when a vehicle is driven off-road or parked. In addition, when such a catalytic converter is subjected to repeated quenching as, for example, when driving through puddles of water, thermal fatigue of the solder joints holding the layers of the honeycomb structure of the metallic monolith together can result. It is, therefore, desirable to mount the metallic monolith in the metallic casing with a mat which provides thermal insulation.
Catalytic converters with ceramic monoliths have a space or gap between monolith and metal casing which increases during heating because of differences in thermal expansion; in the case of catalytic converters with metallic monoliths, this gap decreases upon heating. This is so, even though the thermal expansion coefficients of the metallic monolith and metal casing are similar since the metallic monolith becomes much hotter than the metallic casing resulting in a decreased gap between the two elements. Conventional intumescent mat mounting materials lack the high temperature resiliency needed to continue to provide support for metallic monoliths as the converter is cycled between high and low temperatures.
Prior efforts to produce catalytic converters having ceramic catalyst supports mounted with ceramic fibrous mats include UK Patent Application 2,171,180 A which relates to ceramic and mineral fibrous materials for mounting ceramic monoliths in catalytic converters. The fibrous material is wrapped and compressed under vacuum and sealed in a substantially air impervious plastic envelope or pouch. In use, the plastic will degrade or burn and release the fibrous material so that it expands to hold the ceramic monolith securely.
U.S. Pat. No. 4,693,338 relates to a catalytic converter comprising a ceramic monolith with a blanket of fibers having high resistance to high temperatures between the monolith and the metallic case, the blanket being substantially devoid of binder and devoid of water of constitution and being highly compressed, and a sealing element (gas seal) surrounding the end of the ceramic monolith which is adjacent the outlet of the converter.